[GadgetFsPhy] Added threads to IN endpoints
[GadgetFsPhy] Added threads to IN endpoints

Since write is blocking, and there's no polling on regular endpoints in
gadgetfs, we cannot just send the data on the same thread from which
we read from the control endpoint.

This commit fixes it by creating a separate thread for each endpoint,
regardless of its direction.

nccgroup/umap2,nccgroup/umap2